Eruption Dynamics and Hydrological Implications of Basaltic Volcanic Centres at Lake Taupo

Abstract

The Taupo Volcanic Centre (TVC) is a major rhyolite centre within the Taupo Volcanic Zone, yet a small volume of the centre is comprised of monogenetic basalt volcanoes. Four TVC basalts that lie north of Lake Taupo have been examined in this study: the tuff deposits of Acacia Bay, Kaiapo and Kinloch, and the scoria cone complex of Punatekahi. Inferred ages of the basalts range from ~ 200 ky (Acacia Bay tuff) to lt; 9.5 ky (Kinloch tuff). The particular aim of this study is to determine the volcanological and hydrological controls responsible for the nature of the basaltic deposits through: field description; data on grain size, vesicularity, componentry and clast morphology (including SEM); petrography; and XRF geochemistry of six representative samples. The basalts share a similar mineralogy (plagioclase gt; olivine, augite) and geochemistry (low-K volcanic arc magmas), but can be clearly distinguished by their physical characteristics. The Punatekahi basalt deposit is comprised of thick beds of scoria lapilli, and welded agglutinate, formed by magmatic strombolian and hawaiian eruptions. The Acacia Bay, Kaiapo and Kinloch deposits are all tuff deposits that have a significant base surge component resulting from phreatomagmatic surtseyan and taalian eruptions. The Punatekahi scoria deposit has higher vesicularity values and lower lithic abundance than the other three basalt centres, reflecting the difference between magmatic and phreatomagmatic eruption mechanisms, respectively. All of the four basalt deposits are located in areas that have at one time been covered by Lake Taupo (or its ancestor Lake Huka), and the phreatomagmatic deposits of Acacia Bay, Kaiapo and Kinloch all have evidence of partial subaqueous deposition, suggesting that a lake was present at those sites at the time of eruption. Explosive interaction of magma with groundwater occurred during all of the four eruptions. For the eruptions at Acacia Bay, Kaiapo and Punatekahi, Rangatira Ignimbrite is inferred as a likely aquifer source, and either the rhyolite dome or a fault is posited as the water source for the Kinloch basalt eruption. The Kinloch tuff deposit is younger than the other TVC deposits by at least ~ 100 ky. The presence of this much younger basalt volcano indicates that, despite the dominance of rhyolitic eruptions, basalt volcanism has an ongoing presence in the TVC, and a future basalt eruption, with unique hazards, are a real possibility.